Gordon W. Lorimer

REVIEW ON RESEARCH AND DEVELOPMENT OF MAGNESIUM ALLOYS

The current research and development of magnesium alloys is summarized. Several aspects of magnesium alloys are described: cast Mg alloy, wrought Mg alloy, and novel processing. The subjects are discussed individually and recommendations for further study are listed in the final section.

Precipitation reactions in Magnesium-rare earth alloys containing Yttrium, Gadolinium or Dysprosium

Abstract Precipitation reactions have been evaluated in three magnesium-rare earth alloys, containing yttrium, gadolinium or dysprosium. Differences have been noted in precipitate morphology, volume fraction and formation kinetics. These features have been combined with phase composition data and related to mechanical properties.

Phase compositions in magnesium-rare earth alloys containing yttrium, gadolinium or dysprosium

Abstract Phase compositions have been investigated, using thin foil energy dispersive X-ray spectroscopy, in three magnesium-rare earth alloys, containing yttrium, gadolinium or dysprosium. Compositions are suggested for the as-cast eutectic and β precipitate phases and possible compositions for the β 1 precipitate phases are discussed.

Precipitation (exsolution) in an orthopyroxene

Ion-thinned samples of orthopyroxene from the Stillwater complex, Montana, have been examined in the electron microscope at 100 and 1000 kV. Exsolution lamellae of a calcium-rich, monoclinic pyroxene (augite) up to 0.5 μm in width were observed parallel to (100) of the matrix. The interface between the matrix and precipitate is semi-coherent and consists of a regular network of dislocations. Nucleation occurs at grain boundaries, subgrain boundaries and individual dislocations. Subsequent growth takes place by the movement of ledges along the (100) interface.The matrix orthopyroxene also contains coherent, homogeneously-distributed precipitates (GP zones). A precipitate-free zone occurs adjacent to each augite lamella.

The relationship between the microstructure and microwave dielectric properties of zirconium titanate ceramics

Zirconium titanate (ZrTiO4) ceramics have been prepared by the mixed oxide route using small additions of ZnO, Y2O3 or CuO. Specimens were sintered mainly at 1400 °C and cooled at various rates: water-quench, air-quench, 300 °C h−1, 120 °C h−1, 6 °C h−1 and 1 °C h−1. Products prepared with additives exhibited densities of at least 93% of the theoretical value. As the cooling rate after sintering was decreased, the length of the lattice parameter in the b direction was reduced and transmission electron diffraction revealed superlattice reflections associated with cation ordering. For specimens cooled at 1 °C h−1, electron diffraction patterns exhibited features consistent with an incommensurate superstructure in the a direction. The dielectricQ value of rapidly cooled (air-quenched) ceramics was 2000 at 5 GHz. With an increase in the degree of cation ordering theQ value increased to a maximum of 4400 for specimens cooled at 6 °C h−1. For specimens cooled at the slowest rate (1 °C h−1) theQ value fell to 2000 due in part to the presence of microcracks and exsolved ZrO2. Diffusion of trivalent impurities (yttria) into the host ZrTiO4 grains also led to a lowering of theQ values.The microwave dielectric properties of zirconium titanate ceramics are sensitive to processing conditions and mircrostructural features. The highestQ values (lowest loss) should be achieved in homogeneous specimens, free of trivalent impurities and lattice defects, in which lowQ-value second phases, microcracks and pores are eliminated.

The Recrystallization Behavior of AZ31 and WE43

The deformation and recrystallization behavior of two magnesium alloys, AZ31 and WE43, have been investigated. The cast alloys were heat treated to produce various distributions of second phase particles and deformed in a channel die at a strain rate of 10-4 s-1 at temperatures between 523 and 673 K. The alloys were subsequently annealed at temperatures between 708 and 798 K.The AZ31 and WE43 were extruded at 555 and 633 K, respectively. The microstructures were compared to those developed during channel die deformation. The tensile and compressive strengths and the texture of the alloys were determined in the as-extruded and recrystallised conditions.Optical microscopy and electron backscattered diffraction (EBSD) were used to characterize the microstructures of the alloys. The EBSD technique was used to determine the texture of the deformed and annealed samples.

The identification of asbestos

Techniques are described for identifying and distinguishing between the six varieties of asbestos. When the fibres are large and numerous, as is the case for insulation materials, simple optical or X‐ray methods suffice. For environmental samples one must normally resort to electron microscopic techniques for a positive identification. When asbestiform minerals are to be distinguished from other fibrous silicates it is often necessary to use both electron diffraction and X‐ray spectrometry.

Recrystallization Behaviour of Two Magnesium Alloys

The deformation and recrystallization behaviour of two magnesium alloys, WE43 and an experimental “Alloy A1” containing a large amount of second phase, have been investigated. Optical microscopy and electron backscattered diffraction (EBSD) were used to observe each alloy after deformation in a channel die at 553 and 558 K and subsequent recrystallization during annealing at 673 K. Alloy A1 dynamically recrystallised during deformation at 553 and 558 K. In WE43 the particle distribution affected the onset of dynamic recrystallization. Both alloys exhibited evidence of particle stimulated nucleation (PSN) of recrystallization at second phase particles as well as recrystallization at pre-existing grain boundaries. The A1 and WE43 alloys were extruded at temperatures of 573 K and 633 and 663 K, respectively. The extruded microstructures were compared to those that developed during channel die deformation.

Discontinuous precipitation in cobalt-tungsten alloys

Abstract Discontinuous precipitation in a Co32 wt%W alloy aged in the temperature range from 875 K to 1025 K has been investigated. Philips EM 430 STEM has been used to characterise the microstructure and to measure the composition profiles across individual lamellae of ϵ Co and Co 3 W phases in partially transformed specimens. Two kinds of cellular precipitates have been found in the alloy. The initial transformation product, identified as “primary” lamellae with spacing of a few nanometres is replaced during prolonged ageing by “secondary” lamellae with a much larger interlamellar spacing, typically a few tens of nm. Line scans across cell boundaries of the “primary” lamellae revealed that, just behind the advancing cell boundary, the solute content is far from the equilibrium state. This solute excess within the cells is quickly removed at the ageing temperature. Calculations show that the diffusion process was too rapid to be identified as ordinary volume diffusion. Investigation of the kinetics showed that discontinuous precipitation is controlled by diffusion processes at the advancing cell boundary. This proposal has been confirmed by STEM analysis of tungsten profiles in the depleted ϵ Co lamellae.

An electron-microscopic study of precipitation (exsolution) in an amphibole (the hornblende-grunerite system)

Transmission, analytical and high-voltage electron microscopy have been used to study the distribution of phases in the naturally occurring silicate minerals hornblende and grunerite. These two minerals belong to the amphibole group and previous analytical work has suggested the existence of a miscibility gap between them involving the segregation of calcium. The minerals in the specimens studied are coarse-grained, having crystallized slowly during metamorphism.Large lamellar precipitates are present in both phases. These precipitates form on (100) and (¯101) planes of the matrix and have remained coherent. Steps are present on the broad faces of these lamellae, suggesting that they have thickened by the propagation of ledges as observed in metal alloy systems. These features are common to both phases, but the grunerite matrix contains an additional set of fine, coherent precipitates which have formed at a later stage and show distinct precipitate-free zones around the earlier precipitates. Analytical microscopy has been used to confirm qualitatively that the precipitation involves segregation of calcium.